1. Field of the Invention
The present invention relates to an oxygen sensor element that holds a lamination type ceramic heater, which is applied to an exhaust system of an internal combustion engine to control an air fuel ratio of the engine.
2. Related Art
Conventionally, an air fuel ratio sensor is installed in an exhaust system of an automotive engine to detect an air fuel ratio, and the combustion of the engine is controlled based on the detected air fuel ratio. An oxygen sensor containing an oxygen sensor element is widely employed as the air fuel ratio sensor.
The oxygen sensor element is composed of a cup-like solid electrolyte body having oxygen ion conductivity and defining an air chamber therein, an outer electrode disposed on an outer surface of the solid electrolyte body, and an inner electrode disposed on an inner surface of the solid electrolyte body. The inner surface of the solid electrolyte body is exposed to the air chamber. Further, a ceramic heater is disposed in the air chamber to heat the oxygen sensor element to an activation temperature. A pole-like ceramic heater is typically employed as the ceramic heater.
In recent years, to reduce manufacturing cost, a plate-like lamination type ceramic heater has been suggested as a replacement for the pole-like ceramic heater. For example, JP-A-7-35723, JP-B2-5-2101, and WO 94/14057 disclose this kind of ceramic heater. The lamination type ceramic heater is composed of a plurality of laminated heater substrates, one of which has a heater layer for developing heat, and a lead layer for supplying electricity to the heater layer.
However, the mechanical strength of the lamination type ceramic heater is smaller than that of the pole-like ceramic heater. As mentioned above, the ceramic heater is disposed within the air chamber of the oxygen sensor element. Within the air chamber, the ceramic heater may be swung like a pendulum into contact with the solid electrolyte body due to impact, vibrations, and the like applied externally to the oxygen sensor element. In such a case, the lamination type ceramic heater is easily damaged. The damage to the ceramic heater can involve not only the heater substrates but the heater and lead layers to cause electrical disconnection. In addition, the heater and lead layers can be oxidized by air coming through the damaged portion, resulting in an increase in resistance of the heater and lead layers.